Jan. 31, 2013: Republican Chuck Hagel, President Obama's choice for defense secretary, listens as he testifies before the Senate Armed Services Committee during his confirmation hearing, on Capitol Hill in Washington. (AP)

WASHINGTON – A Senate committee has postponed a vote on former Sen. Chuck Hagel's nomination to be defense secretary after Republicans accused him of withholding financial disclosure information from his time in the Senate.

A letter to Hagel from 23 Republican senators, obtained by Fox News, expresses concern about fees Hagel may have collected for speaking engagements from various groups. The senators, including Senate Majority Leader Mitch McConnell and Minority Whip John Cornyn, argue Hagel did not disclose to the panel which “entities” he has dealt with.

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“Your refusal to respond to this reasonable request suggests either a lack of respect for the Senate’s responsibility to advise and consent or that you are for some reason unwilling to allow this financial disclosure to come to light," the letter reads.

The Senate Armed Services Committee Chairman Carl Levin said in a written statement Wednesday that he had hoped to hold a vote when the committee met on Thursday for a separate hearing on the deadly attack at the U.S. Consulate in Benghazi, Libya, in September.

But Levin said the committee's review of President Obama's nomination has not been completed. He said he would schedule a vote as soon as possible.

Hagel, a former two-term Republican senator from Nebraska, has faced strong opposition from his ex-GOP colleagues, who have questioned his past statements and votes on Israel, Iran and nuclear weapons. It was unclear whether the delay in the vote would derail the nomination or merely postpone action on Obama's choice to replace Defense Secretary Leon Panetta.

In their letter to Hagel, Republicans complained that he failed to answer several questions, including details on all compensation of more than $5,000 that he had received over the past five years. They also had pressed him on his recent speeches, the groups he has addressed and their donors.

Freshman Sen. Ted Cruz, R-Texas, who opposes Hagel's nomination, had complained to his colleagues about the information the nominee provided during a closed meeting on Tuesday. Other Republicans raised objections to a vote.

"I'm not going to make any decision on Sen. Hagel until we get all the information we've requested," Sen. Lindsey Graham, R-S.C., told a small group of reporters Wednesday. "I don't think we should be voting."

Hagel, in a separate letter to committee Republicans, had tried to explain that some of the information they were seeking was not available.

"My role with respect to the entities you identify is as a current and former board or advisory board member. I was not involved in the day-to-day management of any of these firms, and have not been involved with some for the firms for years now," Hagel wrote. "Thus, as a matter of fact, I do not believe I have any of the information requested. More importantly, the information you seek is legally controlled by the individual entities and not mine to disclose."

Senate Democrats, who hold the majority, continue to stand behind the nomination, and no Democrat has said he or she would vote against the president's pick for his second-term national security team. Hagel, 66, is a decorated Vietnam combat veteran.

About a dozen Republicans have said they would oppose their former colleague and several others have indicated they were likely to vote no.

Democrats hold a 55-45 advantage in the Senate, and two Republicans have announced their support for Hagel -- Sens. Thad Cochran of Mississippi and Mike Johanns of Hagel's home state of Nebraska. More than a handful of Republicans, including Sen. John McCain, R-Ariz., have said they oppose a filibuster of the nomination.

Last month, Hagel told Pentagon officials he would divest some of his financial holdings and resign from several corporate boards and public interest groups to avoid potential conflicts of interest if he wins Senate confirmation.

He said he would resign his corporate board post at Chevron Corp. and shed investments in the energy company, a major government contractor. He also would cut ties and investments with the McCarthy Group LLC, an Omaha-based private equity firm.

Hagel also pledged to cut ties with several academic and public interest groups, including Georgetown University and the Atlantic Council.